The present invention relates to a process for making an externally heated cast iron vessel intended for containing reactive molten metals, such as aluminum, resistant to attack by the molten metal; thereby increasing the useful service life of the vessel and minimizing contamination of the melt. The present invention also relates to the vessel thus produced.
In refining molten aluminum and other reactive metals, it is often desirable to use a vessel which is externally heated. Cast iron vessels are desirable because they have high thermal conductivity, can be cast in any desired shape and have a relatively low coefficient of thermal expansion. The problem, however, with cast iron is that it is corroded by molten aluminum. It is well known in the art that aluminum is a powerful solvent in its molten state, and that consequently care must be exercised in selecting materials with which it will come in contact during various processing steps such as melting, alloying, degassing, fluxing, filtration, transfer and casting. Improper selection of such material may cause contamination of the melt by reduction or solution of the container as well as deterioration of the container. It is normal commercial practice therefore to coat cast iron objects which are to be used in contact with molten aluminum, such as with, for example, a wash of red mud, zirconium silicate, mica, iron oxide or titanium oxide. Sodium silicate may be added to the wash coating to improve its adherence to the cast iron. Such coatings are generally applied by brushing or spraying on to these portions of cast iron surface which will come in contact with the melt. However, these coatings wear off easily. The problem of the limited service life for externally heated cast iron vessels used for containing molten aluminum has not been satisfactorily solved by the prior art.